Important steps to be followed for construction of headwork of a Flow Irrigation Scheme :
Generally the headwork is proposed for construction in the meandering loop of a river. If there is no suitable meandering loop, the headwork can be constructed in the riverbed itself by constructing cofferdam upstream as well as downstream of the proposed headwork. A diversion channel is to be constructed for channelization of diverted river water beyond the downstream cofferdam.
The area of excavation of soil for construction of head regulator are to be marked on the plan of the headwork in the drawing. The area covered by the base of abutment walls on both sides and the area covered by upstream and downstream of the inverted filter, boulder launching aprons are considered with at least 2.00 m wide working space. The different depth required for excavation of soil are to be shown in the cross sectional drawing of the headwork to workout of the quantity of earthwork for excavation. Accordingly the excavation is done to the lowest level of the structure with dewatering if required.
After excavation of soil to the required level and area, the Z type sheet piles of specified length are driven at the upstream and downstream of the floor and below abutment wall of the headwork to the designed level.
After driving the sheet piles, RCC pile caps are to be constructed after fitting fixing reinforcement. At least 4 nos. of 16mm dia T.S rods are to be tied horizontally with the stirrups of pile cap. The concreting is to be done after putting shuttering on both sides of the pile cap.
Casting of cement concrete of specified proportion is to done from the deepest level of the downstream floor as per designed drawing. When the base level of abutment wall is attained, the concreting of floor is to be extended on either sides as concrete of base foundation of abutment wall.
After the concreting in the base of abutment wall, the fitting and fixing of reinforcement at the base and abutment walls are to be made as per design and specifications and to be ready for concreting with shuttering.
The C.C casting of glacis and piers can be made simultaneously as per level of designed drawing and the casting is extended towards U/S floor. The C.C inverted filter at U/S and D/S is constructed as per design and drawing after C.C floor is completed. After completion of inverted filter, the U/S and D/S boulder launching aprons are constructed along with boulder sausage, pitching on the side slope of guide and afflux bund as per drawing and design.
At the time of casting of abutment walls and piers above crest level of C.C floor, a deep groove of 40cm × 25cm is to kept for placing the guide channels in the pier and abutment walls. Same size groove is also kept on the crest for fitting the bottom channel.
After attaining the top level of abutment wall and piers, the simply supported gang way slab is to constructed leaving 60 cm gap between the gang way slab and end of guide channel.
The iron shutters are to be designed considering the pond level, crest level of head work. The height of iron shutters is calculated as shown below with given R.L of crest and Pond level.
Suppose pond level=50.00m
And crest level=48.00m
Then Height of shutters = (50.00-48.00) + 0.25 =2.25m
The height of operating platform for iron shutter is determined considering the High flood level, as the shutters are to be lifted and kept above High Flood Level(HFL) for passing high flood discharge.
The level of operating platform will be =HFL+ Height of Iron Shutter+Free space.
Hence height of operating platform from the crest =(53.45-48.00) m =5.45m
Iron shutters are constructed with specified thickness of Iron Sheet fabricated on horizontal Iron Channels of specified size fitted with at least 6 (six) numbers of guide rollers and to be lifted with threaded Iron shaft of minimum 2 ½ dia ,steering fitted on four pod stand as per drawing and pulleys with 16mm dia wire rope fitted with counter weights.